struct jpeg_error_mgr {
/* Error exit handler: does not return to caller */
JMETHOD(void, error_exit, (j_common_ptr cinfo));
/* Conditionally emit a trace or warning message */
JMETHOD(void, emit_message, (j_common_ptr cinfo, int msg_level));
/* Routine that actually outputs a trace or error message */
JMETHOD(void, output_message, (j_common_ptr cinfo));
/* Format a message string for the most recent JPEG error or message */
JMETHOD(void, format_message, (j_common_ptr cinfo, char * buffer));
#define JMSG_LENGTH_MAX 200 /* recommended size of format_message buffer */
/* Reset error state variables at start of a new image */
JMETHOD(void, reset_error_mgr, (j_common_ptr cinfo));
/* The message ID code and any parameters are saved here.
* A message can have one string parameter or up to 8 int parameters.
*/
int msg_code;//取之参考http://www.cnblogs.com/black-mamba/p/6754493.html
#define JMSG_STR_PARM_MAX 80
union {
int i[];
char s[JMSG_STR_PARM_MAX];
} msg_parm;
/* Standard state variables for error facility */
int trace_level; /* max msg_level that will be displayed */
/* For recoverable corrupt-data errors, we emit a warning message,
* but keep going unless emit_message chooses to abort. emit_message
* should count warnings in num_warnings. The surrounding application
* can check for bad data by seeing if num_warnings is nonzero at the
* end of processing.
*/
long num_warnings; /* number of corrupt-data warnings */
/* These fields point to the table(s) of error message strings.
* An application can change the table pointer to switch to a different
* message list (typically, to change the language in which errors are
* reported). Some applications may wish to add additional error codes
* that will be handled by the JPEG library error mechanism; the second
* table pointer is used for this purpose.
*
* First table includes all errors generated by JPEG library itself.
* Error code 0 is reserved for a "no such error string" message.
*/
const char * const * jpeg_message_table; /* Library errors */
int last_jpeg_message; /* Table contains strings 0..last_jpeg_message */
/* Second table can be added by application (see cjpeg/djpeg for example).
* It contains strings numbered first_addon_message..last_addon_message.
*/
const char * const * addon_message_table; /* Non-library errors */
int first_addon_message; /* code for first string in addon table */
int last_addon_message; /* code for last string in addon table */
};jpeg_error_mgr结构体中的函数指针是在调用jpeg_std_error()后被初始化的。可自定义这些函数指针也可使用默认函数。
注意msg_parm支持一个字符串或8个int参数。
jerror.h中包含了一些错误打印宏，下面以ERREXIT1为例进行说明：
#define ERREXIT1(cinfo,code,p1) \
((cinfo)->err->msg_code = (code), \
(cinfo)->err->msg_parm.i[] = (p1), \
(*(cinfo)->err->error_exit) ((j_common_ptr) (cinfo)))
实际应用中遇到了这么个打印“Improper call to JPEG library in state 202”，那么，这句话是怎么打印出来的呢？
cinfo.err = jpeg_std_error(&jerr);
cinfo.global_state = ;
(&cinfo)->err->msg_code = (JERR_BAD_STATE);
(&cinfo)->err->msg_parm.i[] = ((&cinfo)->global_state);
((*(&cinfo)->err->error_exit)) ((j_common_ptr) (&cinfo)); //ERREXIT1(&cinfo, JERR_BAD_STATE, (&cinfo)->global_state);
line3~5是line7的展开式。第5行中的*间接访问操作符可以不用(http://www.cnblogs.com/black-mamba/p/6765231.html)
<jpegint.h>中包含global_state的值：
/* Values of global_state field (jdapi.c has some dependencies on ordering!) */
#define CSTATE_START 100 /* after create_compress */
#define CSTATE_SCANNING 101 /* start_compress done, write_scanlines OK */
#define CSTATE_RAW_OK 102 /* start_compress done, write_raw_data OK */
#define CSTATE_WRCOEFS 103 /* jpeg_write_coefficients done */
#define DSTATE_START 200 /* after create_decompress */
#define DSTATE_INHEADER 201 /* reading header markers, no SOS yet */
#define DSTATE_READY 202 /* found SOS, ready for start_decompress */
#define DSTATE_PRELOAD 203 /* reading multiscan file in start_decompress*/
#define DSTATE_PRESCAN 204 /* performing dummy pass for 2-pass quant */
#define DSTATE_SCANNING 205 /* start_decompress done, read_scanlines OK */
#define DSTATE_RAW_OK 206 /* start_decompress done, read_raw_data OK */
#define DSTATE_BUFIMAGE 207 /* expecting jpeg_start_output */
#define DSTATE_BUFPOST 208 /* looking for SOS/EOI in jpeg_finish_output */
#define DSTATE_RDCOEFS 209 /* reading file in jpeg_read_coefficients */
#define DSTATE_STOPPING 210 /* looking for EOI in jpeg_finish_decompress */
<jerror.c>
该文件中error_exit()->output_message()->format_message()有如此调用关系。